default_targets.md

whenTargetIsDefault

When multiple bindings are available for a given service identifier, we can use one of the following features to resolve the potential AMBIGUOUS_MATCH exception:

• Named bindings
• Tagged bindings
• Contextual bindings
• Default targets

In this section we will explain how to use default targets.

We can resolve an AMBIGUOUS_MATCH exception using a named constraint:

container.bind<Weapon>("Weapon").to(Katana).whenTargetNamed("strong");
container.bind<Weapon>("Weapon").to(Shuriken).whenTargetNamed("weak");

Or a tagged constraint:

container.bind<Weapon>("Weapon").to(Katana).whenTargetTagged("strong", true);
container.bind<Weapon>("Weapon").to(Shuriken).whenTargetTagged("strong", false);

The problem with this solution is that we will have to annotate using the @named("strong")/@named("weak") or @tagged("strong", true)/@tagged("strong", false) every single injection.

A better solution is to use a default target:

container.bind<Weapon>(TYPES.Weapon).to(Shuriken).whenTargetNamed(TAG.throwable);
container.bind<Weapon>(TYPES.Weapon).to(Katana).whenTargetIsDefault();

We can use the whenTargetIsDefault to indicate which binding should be used as default to resolve an AMBIGUOUS_MATCH exception when no @named or @tagged annotations are available.

let TYPES = {
    Weapon: "Weapon"
};

let TAG = {
    throwable: "throwable"
};

interface Weapon {
    name: string;
}

@injectable()
class Katana implements Weapon {
    public name: string;
    constructor() {
        this.name = "Katana";
    }
}

@injectable()
class Shuriken implements Weapon {
    public name: string;
    constructor() {
        this.name = "Shuriken";
    }
}

let container = new Container();
container.bind<Weapon>(TYPES.Weapon).to(Shuriken).whenTargetNamed(TAG.throwable);
container.bind<Weapon>(TYPES.Weapon).to(Katana).whenTargetIsDefault();

let defaultWeapon = container.get<Weapon>(TYPES.Weapon);
let throwableWeapon = container.getNamed<Weapon>(TYPES.Weapon, TAG.throwable);

expect(defaultWeapon.name).eql("Katana");
expect(throwableWeapon.name).eql("Shuriken");